Teknik asas linguistik perbandingan ialah [[kaedah perbandingan]] yang bertujuan untuk membandingkan sistem-sistem fonologi, sistem-sistem morfologi, sintaksis, dan leksikon. Pada dasarnya, setiap perbezaan antara dua bahasa yang berkait harus dapat diterangkan dengan amat munasabah, dan perubahan-perubahan sistematik, umpamanya dalam sistem-sistem fonologi atau morfologi, dijangka amat tetap. Walaupun bahasa-bahasa proto yang dibina semula melalui kaedah-kaedah perbandingan adalah andaian, pembinaan semula ini mungkin mempunyai daya peramalan. Contoh yang paling terkenal ialah cadangan [[Ferdinand de Saussure|Saussure]] bahawa sistem [[konsonan]] [[Bahasa-bahasa Indo-Eropah|Indo-Eropah]] mengandungi [[laringeal ]], sejenis konsonan yang tidak terdapat di mana-mana bahasa Indo-Eropah yang diketahui ketika itu. Hipotesis itu telah diwajarkan oleh penemuan [[bahasa Hittite]] yang terbukti mempunyai konsonan-konsonan yang betul-betul sama dengan konsonan-konsonan yang diramalkan oleh Saussure untuk persekitaran-persekitarannya.
Where languages are derived from a very distant ancestor, and are thus more distantly related, the comparative method becomes impracticable. In particular, attempting to relate two reconstructed proto-languages by the comparative method has not generally produced results that have met with wide acceptance. A number of methods based on statistical analysis of vocabulary have been developed to overcome this limitation. The theoretical basis of such methods is that vocabulary items can be matched without a detailed reconstruction and that comparing enough vocabulary items will negate individual inaccuracies.
The earliest method of this type was [[glottochronology]], which proposed a mathematical formula for establishing the date when two languages separated, based on percentage of a core vocabulary of 100 (later 200) items which are cognate in the languages being compared. Glottochronology has met with continued scepticism, and is seldom applied today. Even more controversial is [[mass lexical comparison]] which disavows any ability to date developments, aiming simply to show which languages are more and less close to each other, in a method similar to those used in [[cladistics]] in [[evolutionary biology]]. However, since mass comparison eschews the use of reconstruction and other traditional tools, it is flaty rejected by the majority of historical linguists.
Such vocabulary-based methods are able solely to establish degrees of relatedness and cannot be used to derive the features of a proto-language, apart from the fact of the shared items of compared vocabulary. These approaches have been challenged for their methodological problems - without a reconstruction or at least a detailed list of phonological correspondences there can be no demonstration that two words in different languages are cognate. However, lexical methods can be validated statistically and by their consistency with independent findings of [[history]], [[archaeology]] and [[population genetics]].
There are other branches of linguistics which involve comparing languages, which are not, however, part of ''comparative linguistics'':
Linguistic typology]] compares languages in order to classify them by their features. Its ultimate aim is understand the [[linguistic universal|universal]]s which govern language, and the range of types found in the world's language is respect of any particular feature (word order or vowel system, for example). Typological similarity does not imply a historical relationship. However, typological arguments can be used in comparative linguistics: one reconstruction may be preferred to another as typologically more plausible.
Language contact| Contact linguistics]] examines the linguistic results of contact between the speakers of different languages, particular as evidenced in [[loan words]]. Any empirical study of loans is by definition historical in focus and therefore forms part of the subject matter of historical linguistics. One of the goals of [[etymology]] is to establish which items in a language's vocabulary result from linguistic contact. This is also an important issue both for the comparative method and for the lexical comparison methods, since failure to recognize a loan may distort the findings.
Contrastive linguistics]] compares languages usually with the aim of assisting [[language learning]] by identifying important differences between the learner's native and target languages. Contrastive linguistics deals solely with present-day languages.